This invention relates generally to trigger circuits and specifically to trigger circuits for use in horizontal deflection systems of cathode ray tube (CRT) monitors.
Monitors are well-known and include horizontal and vertical deflection circuits for developing a raster by scanning an electron beam over a light-emitting-phosphor covered faceplate of a CRT. Video processing circuits modulate the intensity of the scanned electron beam to produce a video display. Color monitors also are capable of providing a chromatic display rather than a monochrome display and include colored light-emitting phosphors, means for assuring that the electron beam impacts the appropriate color phosphor and color video signal processing circuitry. Color monitors may also include graphics control means for generating and displaying alpha-numeric and graphics video data in response to appropriate control signals.
Some graphics control systems use different horizontal and vertical formats and afford different degrees of resolution. For example, three vertical resolutions, specifically either 350, 400 or 480 vertical lines, are available in some new monitors. Such systems provide for format or mode control signals which may be encoded by the polarities of the accompanying sync pulses. Thus by selecting different polarities of the horizontal and vertical sync pulses, the mode or format of the accompanying data may be determined. In these arrangements, the monitor must be capable of decoding the incoming sync pulses to determine the display format for the data (and any other changeable parameters that may be involved). It is also incumbent upon the monitor to deliver the appropriate polarity of sync signals for which the monitor is designed to operate. The monitor must, therefore, be capable of decoding the incoming sync pulses and producing sync signals of appropriate timing and polarity for controlling its deflection circuits.
In one commercial monitor, a pair of monostables is utilized for enabling internal control of the video display with respect to horizontal deflection. A first monostable is used to introduce a one line delay to permit a phase control to be incorporated for horizontally shifting the video display on the viewing screen. The output of the pair of monostables is applied to a trigger input of a phase lock loop (PLL) that free runs near the horizontal line frequency and which is keyed by the monostables. That circuit is designed for use with and accepts only one polarity of horizontal sync pulses. With the advent of the new computer graphics systems which provide for encoded, that is, either positive or negative polarity horizontal sync pulses, a need arose for a circuit to enable operation of the monostable with either polarity of horizontal sync.